Known from the state of the art are numerous thermometers that have a protective tube inner part as described above. Thus, such thermometers have a protective tube, into which a sensor, also called a measuring transducer, can be inserted. The thermometer can be, in such case, a temperature sensor, for example, in the form of a thermoelement, such as a thermocouple, or a temperature dependent, resistance element. Known from the state of the art are, additionally, thermometers, in the case of which the measuring transducer is part of a measuring insert, especially a replaceable measuring insert, which is arranged in the protective tube. The measuring insert is composed, in such case, of a protective tube inner part and the measuring transducer, wherein the protective tube inner part serves, among other things, as the holder of the measuring transducer. The protective tube itself serves, in such case, often as protection against, for example, an abrasive, measured material and is inserted into a containment, such as, for example, a tank or a pipe, in which the measured material is located. Such thermometers intended for use in an industrial environment are sold by the firm, ENDRESS+HAUSER ®.
Problematic in the case of such construction is, however, heat transfer between the process and the process environment brought about by the thermometer. Measurement error can result from this. Known from the state of the art is to resist this heat transfer by using, for example, a long installation depth, i.e. a long protective tube length.
Thus, for example, patent DE 102006034246 B3 describes a temperature sensor for a resistance thermometer, which has a metal jacket and is filled with ceramic or mineral material. This resistance thermometer has, however, a considerable thermal drain effect, which can lead to a defective temperature measurement.
Patent DE 19707844 A1 provides external design elements, with which sensor connection lines can be heated or cooled, in order to avoid measurement error arising from thermal drain. The proposed measures require, however, the use of auxiliary energy.
Patent DE 4123093 C2 describes a thermometer constructed of corrosion resistant materials, such as glass, with pressure- and corrosion resistant feedthroughs, wherein a provided assembly head, however, interferes with evacuation of the protective tube volume. Likewise, utility model DE 8108694 U1 discloses a non evacuable thermometer for measuring surface temperatures. The construction proposed in the aforementioned documents leads, moreover, to increased heat transfer, i.e. heat transfer via the protective tube and the measuring insert into the environment of the process. Furthermore, such construction exhibits only poor thermal response to temperature changes in the process.
Utility model DE 7911898 U1 describes a non evacuable measuring gauge, which uses metal insert parts and, consequently, likewise tends to exhibit thermal drain.
Laid open DE 1118996 relates to a thermometer, which has a glass feedthrough containing a bore and serving as insulation piece. This thermometer uses a pressure-resistant protective tube with neck tube and screw fittings and has a measuring gauge, which is embedded in synthetic resin and therewith also tends to exhibit thermal drain.
Patent DE 859066 discloses a heat insulating guide, on which a spring abuts, wherein thermal contact should be achieved by means of pressure against the measuring element, wherein, however, also here, thermal drain effects are present due to the utilized materials.
Patent DE 3721983 A1 discloses an immersion thermometer, in the case of which sufficient thermal contact with the measured medium can be achieved exclusively through a long installation depth.
Patent DE 4138460 C2 describes an evacuable thermometer, which, however, uses a thermally draining, internal holder. The volume surrounded by the protective tube is, indeed, evacuable, but the proposed holder of transparent ceramic produces thermal drain that disturbs the measuring.
Provided in Patent EP 1831504 B1 are thermally insulating layer arrangements, which, however, also tend to exhibit thermal drain.
Patent DE 3330491 C2 describes a thermocouple with two wires accommodated with tension relief in a protective sleeve. The wires are of different materials, such as, for example, iron and constantan, which are connected conductively together at the end toward the measuring point to form a thermocouple and, at least in the region of the measuring point, are surrounded by a cup of heat resistant material. However, thermal drain occurs also in the case of this construction.
Patent DE 576734 utilizes a needle shaped thermocouple, wherein therocouples in the form of hollow needles are provided, which have, however, only limited mechanical strength and their thermal drain characteristic presupposes an immersion depth amounting to at least 6-times the probe diameter.